Sodium vapor discharge lamps are one of the most significant products of lighting industry. due to their outstanding specific lumen/watt performance ratio they are becoming increasingly popular in energy saving public lighting applications. The expectable life of lamps of the type produced at the present time does not exceed 10 to 15 thousand hours, depending on the type involved. Because of the high cost of such lamps an ever increasing life expectancy is demanded by the market, reaching or even exceeding 20 to 25 thousand service hours, for the reduction of operating costs.
According to the known manufacturing technology for sodium vapor lamps, such as is described in Hungarian patent specification No. 178,880, first one end of a discharge tube is sealed in a vacuum furnace. Then the discharge tubes are transferred into a high-purity glove box in which the adding of dopants takes place. After doping, the tubes are re-transferred into the vacuum furnace, where they are filled with gas and the other ends of the tubes are sealed. This method is used in the case of discharge tubes which have no suction pipes attached to them. The manufacturing method is also similar in the case of discharge tubes with attached suction pipes, but with the difference that the dopants are added only after both ends were sealed and the pumping, gas filling and shutting-off steps are then performed in a multi purpose apparatus. The sequence of manufacturing steps and the design of the sealing furnaces and pumps depend on the specific model of the discharge tube, such as is described, for example, in U.S. Pat. Nos. 3,363,133; 3,363,143; and 3,609,437.
All known methods for making gas discharge tubes have a common feature that the discharge tubes are exposed several times to the free atmosphere during the intervals between various manufacturing steps. It is a recognized fact that the most important factor that affects useful life of lamps is the purity of their gas filling (i.e. partial pressure of water and of oxygen) in the discharge tube. The gas purity obtainable with currently known manufacturing technologies, i.e. the transfer of tubes in air between operations is unsatisfactory because the average oxygen and water concentration being in the range of 100 to 500.times.10.sup.-6 g/g, whereas to increase useful life and reliable production the internal gas purity should be kept below an average concentration of 10.times.10.sup.-6 g/g.